Amplifier circuits with gains are well known in the art. See for example, U.S. Pat. Nos. 4,366,450; 5,355,096; 5,661,437; 7,046,081; 6,906,595; 7,332,964; 6,522,195; 6,977,552; 7,423,487; and USP publication 2009/0015334. Typically, the prior art amplifier circuit comprises an amplifier with a feedback circuit. The feedback circuit can be with either active elements or with passive elements. One prior art amplifier circuit with an amplifier and an active feedback circuit is shown in FIG. 9 of U.S. Pat. No. 5,661,437. The problem with this amplifier circuit is that the variable gain step is as small as 14 dB. The amplifier circuit has very low gain due to the fact that both of the high gain and low gain have negative values of −2.5 dB and −16.5 dB respectively. Another prior art amplifier circuit with an amplifier and a bias circuit with switches as the active feedback elements is shown in FIG. 1 of U.S. Pat. No. 6,977,552. In this prior art circuit, the manufacturing cost is believed to be high because there are two types of transistors used. The gain step of the amplifier circuit is also limited by the gain of the amplifier, because the RF signal is only bypassed by the switches at a low gain mode. Moreover, the output power ability and linearity are limited by the switches.
It is therefore desirable to have an amplifier system with a high step gain with reduced current consumption, with little or no distortion to RF signal transmission at low gain state.